Provide a good error message when python3 is a script on MacOS

[facundominguez]

If python3 is a script, running cabal_wrapper fails on MacOS because the shebang in cabal_wrapper is ignored. e.g.

Use --sandbox_debug to see verbose messages from the sandbox
from: can't read /var/mail/__future__
from: can't read /var/mail/__future__
from: can't read /var/mail/__future__
bazel-out/host/bin/external/rules_haskell/haskell/cabal_wrapper: line 13: import: command not found
bazel-out/host/bin/external/rules_haskell/haskell/cabal_wrapper: line 19: del: command not found

rules_haskell could check this when generating the shebang and produce an error message that points the user to the cause of the problem. Additionally, maybe it could check that python3 has executable permissions.

Here's more context tweag/rules_nixpkgs#186. And here's were the python3 executable is searched for

rules_haskell/haskell/private/workspace_utils.bzl

Line 36 in 26494b7

def find_python(repository_ctx):

.

I've noticed also that the shebang works when cabal_wrapper is run from the terminal, but it doesn't work when called from another script like

$ cat test.sh
#!/bin/bash
bazel-out/host/bin/external/rules_haskell/haskell/cabal_wrapper